There are varieties of methods of conveying a conveyed object such as a work, including a method of conveying the work while carrying it on a belt conveyer, a method of conveying the work while transferring the work using an index table or a robot arm equipped with a suction nozzle to fix the work by vacuum suction, and a method of conveying the work by a combination of conveyance mechanisms of the index table and the robot arm.
Conventional belt conveyer system has a problem of being incapable of conveying the work in a stable seating mode for the case of conveying a cylindrical work having a large length relative to its diameter and having a small diameter because that kind of work is unstable on the belt conveyer. When image inspecting the work at the entire outer periphery thereof, if the seating pitches of the works on the belt conveyer are small, the image processing is difficult because other works enter the visual field of the imaging camera and because accurate image inspection cannot be given under an influence of reflection of lighting on other works. Consequently, the works are requested to be seated while securing a relatively wide distance therebetween. Responding to the situation, increasing the processing speed of the image inspection requires increasing the belt speed, which raises a problem that cylindrical works which have a large length relative to the diameter and which have a high center of gravity cannot be seated stably. In particular, it has been impossible to convey such works with a small diameter.
Regarding the conventional conveying methods using the index table or the robot arm, the suction nozzle is required to be selected to agree with the shape and size of the works. Accordingly, switching the works at the change of products needs to stop the operation of the apparatus for the replacement of the suction nozzle, which raises a problem of deteriorated operation efficiency. In addition, there is a problem of increase in the apparatus cost because of the large number of parts and their complexity. Furthermore, since the suction nozzle hides a portion of the work, there is required a step of re-catching the work at a different place, which further increases the number of parts of the apparatus and increases the cost of apparatus.
For the index-table method, increasing the processing speed of the image inspection requires that the suction speed of the suction nozzle and the operating speed of the index table itself are increased. However, there are problems that a sufficient suction time should be secured using a differential vacuum gauge to ensure a correct operation of vacuum suction responding to the material of and surface treatment state of the work, and that a limitation exists in increasing the operating speed because the diameter of the index table increases owing to the installation of the imaging camera and thereby the moment of inertia during the rotation of the index table is limited. Furthermore, there is a problem that, when the suction nozzle is equipped with a mechanism to rotate the work, the weight-increase deteriorates the moment of inertia, which increases the operating time. If a mechanism is adopted wherein the suction nozzle is rotated by a separate driving source after the index table is rotated, an operating time is added. For the conveying method using the robot arm, the time for returning the robot arm is required so that there is a structural problem that increasing the operating speed is limited.
To solve these problems, there has been proposed a roller-conveying apparatus which is illustrated in FIGS. 7(a) and (b), (refer to, for example, Patent Document 1), in which a spiral lead 52 is fanned in a convex shape at a constant pitch on the outer periphery surface of a roller 51 a which is one of two rollers 51a and 51b arranged in parallel with each other, and each of the rollers 51a and 51b is rotated to convey a conveyed object 50 such as a work which is held between the rollers 51a and 51b. According to the roller-conveying apparatus, increasing the rotational speed of each of the rollers 51a and 51b easily increases the processing speed of image inspection in a state that the conveyed object 50 is held between the rollers 51a and 51b. 
[Patent Document 1] Japanese Patent Application Laid-Open No. H10-221311
The roller-conveying apparatus described in Patent Document 1, however, conveys the conveyed object by the action of contact with the convex lead of the roller, and the contact results in stopping of the rotation of the conveyed object or lifting up of the conveyed object to generate a phenomenon wherein the conveyed object is vibrated. Consequently, accurate image inspection cannot be performed. In addition, there is a problem that the contact between the conveyed object and the convex lead scrapes the surface of the conveyed object to generate scraped debris.
The present invention was perfected to solve the above problems. An object of the present invention is to provide a conveying apparatus and a conveyed object inspection apparatus capable of performing accurate image inspection and preventing the generation of scraped debris of the conveyed object.